Authorities say they are investigating it as an act of terrorism. A state of emergency has been declared in Florida and in the city of Orlando "because of the scale of the crime," Mayor Buddy Dyer said.

Florida Governor Rick Scott told reporters the attack was "clearly an act of terrorism," adding: "It is sickening, this should make every American angry."

The gunman, who is among the dead, has been named as 29-year-old Omar Mateen, a US citizen of Afghan descent.

Florida Gov has declared a state of emergency. City of Orlando also declared state of emergency.

The attacker was involved with a three-hour stand-off with officers in the Pulse club before SWAT teams stormed the building and shot him dead.

He was described by an FBI Special Agent as having "leanings" towards extremism and Islamic State. NBC news is reporting that Omar Mateen called 911 before the attack and swore his allegiance to the leader of terror group Isis.

Confirming that the number of dead had risen from 20 to 50, Mayor Dyer told a press briefing: "Today we're dealing with something that we never imagined and is unimaginable.

"There's blood everywhere".

Armed with a device thought to be a bomb, an assault rifle and a handgun, Mateen opened fire in the busy club in the early hours of Sunday morning before taking hostages.

A member of the Orlando Police Department was shot during the incident, but has survived thanks to his Kevlar helmet.

Pulse shooting: In hail of gunfire in which suspect was killed, OPD officer was hit. Kevlar helmet saved his life. pic.twitter.com/MAb0jGi7r4

An Islamic State-affiliated Twitter account posted an image of a man it claims to have been the shooter, and the group later claimed responsibility for the attack.

In a statement, House Homeland Security Chairman Michael McCaul said: "This appears to be the worst terrorist attack on American soil since 9/11, and it is a sobering reminder that radical Islamists are targeting our country and our way of life."

Other officials have cautioned, however, that a Islamist motive requires further investigation.

US Congressman Alan Grayson said the theory that this was "an act of international terrorism" is "much more speculative at this point".

He said authorities are investigating whether the attack was a "hate crime perpetrated against gays."

He previously told reporters he thought it was "no coincidence" the shooter targeted a gay bar, adding that there is currently no "direct information" to suggest the gunman was associated with any particular group.

The gunman's father has also suggested it was a homophobic attack, telling US media it had "nothing to do with religion".

"He saw two men kissing each other in front of his wife and kid and he got very angry," Mir Seddique told NBC News.

Barack Obama was briefed on the shooting by his counterterrorism aide this morning and a White House statement said: "Our thoughts and prayers are with the families and loved ones of the victims."

Speaking in the White House the President said “We pray for the families who are grasping for answers... We stand with the people of Orlando, who have endured a terrible attack on their city.”

The attack unfolded just after 2am when an officer working as a security guard at the club raised the alarm and engaged in a gunbattle with Mateen. The gunman then took a number of hostages inside the popular gay club.

Some clubbers mistook the sound of gunshots for music and the bar posted on its Facebook account: "Everyone get out of pulse and keep running."

At 5am police launched a mission to rescue the hostages and nine officers stormed the club, shooting dead the attacker following a short gunfight.

A mother has told how her son texted her from the club toilets where he was hiding, telling her "I'm gonna die" and "Mommy I love you" before a final text saying: "He's coming."

A surgeon at Orlando Regional Medical Centre has warned that the death toll is likely to climb.

Ricardo Negron was in the club when the shooting began.

He told Sky News: "At around 2am someone started shooting. People just dropped on the floor. I guess the shooter was shooting at the ceiling because you could see all the glass from the lamps falling.

"There was a brief pause in the shooting and some of us just got up and ran out the back."

He said he heard "non-stop firing" which probably lasted less than a minute but felt like a lot longer.